This invention relates to a circuit arrangement with an electronically controllable transfer characteristic, in particular a control circuit, in which an amplifier with controllable negative feedback is capable of receiving an input signal, which amplifier has an output connected to a voltage-divider circuit with a frequency-dependent characteristic, which circuit has a plurality of taps connected to an inverting input of the amplifier via a first electronically controllable switch and to an output of the arrangement via a second electronically controllable switch.
Such a circuit arrangement is known from German Patent Specification 31 32 402, which corresponds to U.S. Pat. No. 4,439,739 (Mar. 27, 1984). In this prior-art arrangement the frequency response to the input signal is influenced by suitably controlling the two electronically controllable switches. The two switches are controlled in a manner such that for a boost of the input signal in a frequency band determined by the frequency-dependent voltage divider, the second switch is set to that position in which the signal of the highest level is coupled out. The degree of boosting is changed by setting the first switch to its various positions. Conversely, the frequency range is attenuated in that the first switch is set to the position for maximum negative feedback and the second switch is actuated between its various positions. Particularly when used in a frequency-dependent configuration for, for example, equalizers or tone controls, this circuit arrangement has the drawback that depending on the selected boost or attenuation always a specific frequency band of the same width is influenced. Only the degree of boosting or attenuation is changed. Put in different terms, this means that for comparatively small boosts or attenuations the filtering provided by the circuit arrangement exhibits a smaller Q-factor than for stronger boosts or attenuations in the relevant frequency band. Here, the Q-factor is to be understood to mean the ratio of the frequency at the maximum boost or attenuation to the frequency difference between the frequencies for which the boost or attenuation is 3 dB below the maximum value.